Strabismus is an extraocular muscle disorder resulting in misalignment of the eyes. It is reported to affect one to three percent of the population in the United States (Helveston E. M., "The Incidence of Amblyopia ex Anopsia in Young Adult Males in Minnesota in 1963" Am. J. Ophthal., 60:75-77, 1965; Florm McNeomaier, RW, "Prevalence of Amblyopia" Public Health Rep. 8:29:34, 1966). Ophthalmic photographers are often asked to document this ocular misalignment both before and after strabismus surgery. The resulting photographs are then used for comparative studies, case presentations, teaching tools, and publications. It is thus important to generate accurate and reproducible photographs on a wide range of patients of all ages and ethnic backgrounds affected with strabismus. The present invention is directed to a gaze fixation device for strabismus photography (nine gaze cardinal photography) and deviometry which is referred to herein as the photodeviometer.
Deviometry, the measurement of strabismus in the cardinal positions of gaze, was initially developed as a method of documenting incomitance of strabismus, particularly in the case of a paretic vertically acting muscle.
The first deviometer, the Owen's deviometer (3), was designed in 1947 and consisted of a rotating arm (15.4 cm long) with a near fixation light set at 35.5 cm away from the patient. The angle of fixation remained at 25.degree. from the primary position throughout the different positions of gaze. The deviation was then determined objectively by using the prism and cover test. The major disadvantage of the Owen's deviometer was the lack of an accommodative fixation target. Since adequate control of accommodation was not possible, inaccurate and variable measurements resulted.
Methods other than deviometry for documenting gaze incomitance have been described. The most common clinical method is to passively turn the patient's head so that the eyes are in the intended positions of gaze. The problem with this technique is the difficulty in obtaining consistency of head position. A slight head tilt, chin elevation or depression can easily be introduced. This method is rarely satisfactory because of inaccuracy and inconsistent measurements.
An ideal deviometer would allow measurements in the cardinal positions of gaze while the patient's head remains stationary. Also, it would be highly advantageous to have a central fixation target or images and accommodative fixation target or images set for deviometry or measurements and fixation targets or images set for extraocular muscle dysfunction, such as 25.degree. and 34.degree., and the position of the recording devices, for example, a 35 mm single lens reflex (SLR) camera or a video camera. Such a photo-deviometer fulfills and meets all of these requirements and allows accurate and reproducible pre and post treatment measurements and/or photographs.